A wireless communication network includes at least one Radio Access Network (RAN), which establishes wireless communications between a fixed network node (known as a base station, NodeB, or eNodeB) and a plurality of possibly mobile wireless devices, which may for example comprise machine-type communication (MTC) devices, machine-to-machine (M2M) devices, or user communication terminals, also known as User Equipments or UEs. For the simplicity of explanation and without loss of generality, the RAN is described herein in the context of an eNodeB communicating with UEs, although, of course, other types of devices are in fact included.
The RAN implements these communication links by modulating information onto a Radio Frequency (RF) carrier signal, referred to as a “carrier” or “component carrier.” Network operators license frequency spectra from regulatory bodies, and operate RANs within specific frequency bands over certain geographic areas (cells). Within each band, a carrier raster having a predetermined frequency spacing is defined; RF carriers for uplink and downlink communications are confined to frequencies on this grid. Within each band, a limited number of carriers are operated, to facilitate efficient cell search by UEs.
When a UE first connects to a network, it does not have knowledge of the frequencies of the downlink RF carrier(s) transmitted by the network. The UE performs a cell search by scanning the possible RF carrier positions (that is, the frequency positions on the carrier raster) and tries to identify and synchmnize to any available downlink carriers. Once connected to a network, cell search also must be performed periodically by the UE to provide mobility, since it needs to find and identify adjacent candidate cells for handover. The larger the number of possible carrier positions, the longer the time cell search will take. For this reason, the number of carrier positions in the carrier raster is limited.
As an example, the LTE RAN (also called E-UTRA) has a carrier raster within each operating band, with a 100 kHz spacing between possible carrier frequency positions (f), as depicted in FIG. 1. See also 3GPP TS 36.101 and TS 36.104, the disclosures of which are incorporated herein by reference in their entireties. This 100 kHz grid gives a reasonable number of points on which to search, and in addition matches the block assignment of spectrum licenses awarded to operators, which very often has a granularity of 100 kHz.
The E-UTRA utilizes Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, and OFDM precoded by a Discrete Fourier Transform (DFT) in the uplink. For the purposes of this disclosure, the uplink may also be considered to be OFDM. OFDM is characterized by the transmission of a large number of relatively narrowband subcarriers, which are mutually orthogonal. The overall set of subcarriers is referred to as a “carrier” and the mutual orthogonality means that ideally the subcarriers do not cause interference to each other after demodulation. Another property of OFDM is that it is possible to use a computationally efficient Fast Fourier Transform processing for the receiver and correspondingly an Inverse FFT (IFFT) for the transmitter.
FIG. 2 depicts a representative frequency-domain structure for an LTE system, where the subcarriers are spaced 15 kHz apart. By convention, the frequency of a carrier or subcarrier is its center frequency. In the downlink, the set of subcarriers is centered on the downlink carrier frequency. The center subcarrier is typically unused, to avoid interference from e.g. the Local Oscillator at the DC frequency in the receiver. This subcarrier is thus referred to as the DC subcarrier. In the uplink, the carrier is centered between the two center-most subcarriers. The uplink is usually an even number of subcarriers due to the use of an even FFT size in the UE.
The demand for higher peak data rates and overall throughput continues to increase as wireless networks and UE devices become more sophisticated, and users desire to access more content and services. One way to support this is to use RF carriers having a larger bandwidth. For OFDM using the same subcarrier definition, this means more subcarriers and unpractically large FFTs. An alternative approach to achieving a more wideband signal is to use multiple component carriers that am aggregated and jointly used for communication. This technique is called carrier aggregation.
FIG. 3 depicts an example of carrier aggregation. All component carriers must be placed on the same 100 kHz grid, in order for UEs to be able to use cell search procedures to synchronize to the download carriers, for both initial access and handover. An additional restriction on the carrier spacing is that it must be a multiple of the subcarrier spacing (15 kHz in the LTE example), because the transmitter and receiver should be able to use the same FFT for the aggregated carriers, and to maintain orthogonally between the subcarriers of the two component carriers.
Hence, the component carrier spacing must be a multiple of both the carrier raster spacing (100 kHz) and the subcarrier spacing (15 kHz). In the LTE example, this leads to a carrier spacing which must be a multiple of 300 kHz, which is the lowest common multiple of the carrier raster spacing and the subcarrier grid spacing. The reduced freedom of selecting the component carrier placement results in an unused, and hence undesirable, gap between the component carriers, shown as reserved subcarriers in FIG. 3.
This restriction of RF carrier placement—that the RF carrier center must be on the carrier raster and also to have the subcarriers between component carriers aligned—raises several issues. For a system such as LTE that uses carrier aggregation, there will be a severe limitation on the location of the RF carriers, since both carriers' center frequencies must be on both the RF carrier raster and a common subcarrier spacing.
When designing a system and selecting the RF parameters, these limitations also restrict the choice of the subcarrier spacing. The subcarrier spacing cannot be selected to give a too large “lowest common multiple,” since that would severely restrict the placement of the component carriers.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches descried in the Background section could be pursued but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.